У Вас тута какой-то смесь бульдога (Gulp) с носорогорм (webpack).
Вот пример чистого Gulp4
gulpfile.coffeesass = require 'gulp-sass'
slim = require 'gulp-slim'
coffee = require 'gulp-coffee'
notify = require 'gulp-notify'
plumber = require 'gulp-plumber'
connect = require 'gulp-connect'
gulp = require 'gulp'
paths =
app: 'app'
sass: 'sass/**/*.scss'
slim: 'slim/**/*.slim'
coffee: 'coffee/**/*.coffee'
js: 'js/**/*.js'
########################################
########################################
tsass = ->
gulp.src paths.sass
.pipe plumber { errorHandler: notify.onError message: "<%= error.message %>", title: "Ошибка" }
.pipe sass outputStyle: 'expanded'
.pipe gulp.dest paths.app
.pipe connect.reload( )
########################################
tslim = ->
gulp.src paths.slim
.pipe plumber { errorHandler: notify.onError message: "<%= error.message %>", title: "Ошибка" }
.pipe slim pretty: true
.pipe gulp.dest paths.app
.pipe connect.reload( )
########################################
tjs = ->
gulp.src paths.js
.pipe plumber { errorHandler: notify.onError message: "<%= error.message %>", title: "Ошибка" }
.pipe gulp.dest paths.app
.pipe connect.reload( )
########################################
tcoffee = ->
gulp.src paths.coffee
.pipe plumber { errorHandler: notify.onError message: "<%= error.message %>", title: "Ошибка" }
.pipe coffee bare: true
.pipe gulp.dest paths.app
.pipe connect.reload( )
########################################
twatch = ->
gulp.watch paths.sass, tsass
gulp.watch paths.slim, tslim
gulp.watch paths.coffee, tcoffee
gulp.watch paths.js, tjs
########################################
tconnect = ->
connect.server
root: paths.app
port: 8888
livereload: true
########################################
watch = gulp.parallel tconnect,
gulp.series( gulp.parallel( tslim, tsass, tcoffee, tjs ), twatch )
gulp.task 'default', watch